Method for error handling

ABSTRACT

In a method for error handling in transmission of a datum over a communications system, at least two data words consisting of bits are generated for the datum in accordance with a predefined coding rule, and one of the generated data words is selected taking into consideration a running digital sum formed over the corresponding data word, and the running digital sum of the selected data word is used for the formation of a first running digital sum. The selected data word is converted into a code data word, and a bit of the data word is in each case assigned a two-bit string with two different single-bit values. The code data word and the first running digital sum are transmitted. The received code data word is examined to ascertain whether an erroneous two-bit string exists, in which case the error is corrected using the first running digital sum.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and a system for errorhandling in transmission of a datum.

2. Description of Related Art

In a known method for error handling in transmission of coded data overa communications system, a data word is selected for the data inaccordance with a predefinable coding rule, the data being representedas bits which may assume two different values, ones and zeros. Provisionis made in that case for at least one running digital sum (RDS) to beformed in such a manner that a summed difference of a total number ofthe ones and a total number of the zeros is formed at least over thedata word, and that running digital sum is transmitted preferablyperiodically after a given number of transmitted data words.Furthermore, the running digital sum is re-calculated and periodicallycompared in a receiver on the basis of the data words transmitted. Inthe event of any deviation which may occur, it is possible to recognizean error.

A BRIEF SUMMARY OF THE INVENTION

The method according to the present invention provides for errorhandling in transmission of a datum over a communications system, whichmethod enables recognition and correction of single-bit errors. In afirst step, at least two data words consisting of bits are generated forthe datum in accordance with a predefined coding rule. The data wordsthus generated represent the at least one datum in a character stringthat may have two clearly distinguishable bit values, for example zerosand ones.

It is provided that, for each of those data words, in a second step arunning digital sum (RDS) is calculated. To calculate a running digitalsum it may be provided that a first sum is formed by way of a totalnumber of the first of the two possible bit values, for example the one,within the data word and a second sum is formed by way of a total numberof the second of the two possible bit values, for example the zero,within that data word and a difference between those two sums iscalculated. The difference thus provides information on a frequency ofthe two different bit values within the data word.

Taking into consideration the running digital sums of the data wordsgenerated for the datum, a data word is selected. A first runningdigital sum corresponds in this case to the running digital sum formedover the selected data word or it is calculated by addition of therunning digital sum of the selected data word to a currently givenrunning digital sum. The data word may be selected on the basis that thefirst running digital sum becomes as small as possible in magnitude. Ina third step, the selected data word is converted by a modulation methodinto a code data word. In that operation, a bit of the data word is ineach case assigned a two-bit string with two different bit values. Thecode data word thus provided accordingly has twice as many characters asthe selected data word. The different bit values of the code data wordmay similarly be zeros and ones. Depending on the nature of themodulation method, a two-bit string of the code data word has the twopossible bit strings “10” or “01”.

In a further step, the code data word and the first running digital sumare transmitted, for example periodically. Such a transmission iscarried out between individual stations connected to one another via thecommunications system. In this operation, a first station configured asa transmitter may transmit the running digital sum and the code dataword to at least one second station configured as a receiver.

In a subsequent step of the method, the code data word received by theat least one second station is examined to ascertain whether it has atwo-bit string with two identical bit values, for example two zeros ortwo ones. If that were to be the case, one bit of the code data wordwithin a two-bit string would have been corrupted during thetransmission. Thus, it is possible to identify an erroneous two-bitstring. That erroneous two-bit string may be corrected using or with theaid of the first running digital sum.

In one possible embodiment of the method, it is provided that the dataword for the datum is selected from two possible data words. For thatpurpose, the datum is assigned, in accordance with the predefined codingrule, a first data word and a second data word, these two data wordseach being the inverse of the other. This means that a bit at oneposition of the first data word is assigned a first of two possible bitvalues, and a corresponding bit at the same position of the second dataword is assigned a second of two possible bit values. Whether the firstor the second data word is selected may be determined by a specificcondition, such as, for example, that the first running digital sum isto have as small a magnitude as possible. The first running digital summay in this case directly correspond to the running digital sum of thedata word to be selected or to the sum of that running digital sum ofthe selected data word with a currently given running digital sum. Avalue for the currently given running digital sum depends, for example,on a running digital sum of at least one predecessor data word of thedata word to be selected.

In accordance with a further embodiment of the method, each code dataword received is converted, by a demodulation method corresponding tothe modulation method, that is, the reverse modulation method, in theevent of the occurrence of an erroneous two-bit string into at least twodata words. For these data words provided in that manner, a seconddigital sum is calculated in each case. The calculation of the secondrunning digital sum is carried out in accordance with the calculation ofthe first running digital sum. The respective second running digital summay accordingly, for example, correspond to the running digital sum ofthe respective demodulated data words or be formed from the sum of therespective running digital sum and a currently given running digitalsum. On comparison of the respective second running digital sums withthe received first running digital sum, the error may be corrected,since the second running digital sum corresponds to the first runningdigital sum for precisely only one of the data words. Thus, it ispossible for an original datum to be reconstructed in a clear-cutmanner.

If one assumes, for example, that only one bit in the code data word hasbeen corrupted, on the basis of the modulation method, according towhich a two-bit string is “01” or “10”, it is immediately clear that areceived two-bit string is either “00” or “11”. With the aid of theabove-mentioned coding rule and the first running digital sum acorrection is made; the erroneous two-bit string is assigned the correcttwo-bit string “10” or “01”.

Whether a two-bit string within the code data word has a single-biterror may accordingly be determined solely on the basis of a string ofbit values, that is to say, on the basis of the structure of the dataword. Which of the two single bits within that two-bit string iserroneous may be established by comparison of the first with the secondrunning digital sum. On the basis of the running digital sum it is knownhow often the first of the two bit values and how often the second ofthe two bit values occurs within the data word.

In principle, the datum itself may be coded into bit values which, inconjunction with the coding rule and the running digital sum (RDS), makeerror recognition possible. This is particularly the case when a numberof different data words m is smaller than a variant number 2^(n) ofpossible code data words with n bits.

The method may also be carried out for a group of data. In thatinstance, it is conceivable for two data words to be generated for eachdatum of the group. Which of those two data words will in each case beselected for further processing for the datum may in turn depend on avalue of the first running digital sum. The first running digital summay be so calculated that it is obtained as the sum over all runningdigital sums of all data words to be selected of the group and, forexample, is to be as small as possible in magnitude. The data word for adatum is selected, for example, in dependence on the currently givenrunning digital sum between the two data words available for selection.In this case, the currently given running digital sum corresponds to anintermediate value for the first running digital sum calculated for thedata words so far selected within the group. Thus, it is possible tocause the running digital sum to become, over the group of data words,on average as far as possible zero. That measure allows a substantiallydc-free code to be obtained.

Using the method according to the present invention, it is possible todiscover single-bit errors. With information about an error position,correction of hitherto-ambiguous errors that do not permit any clear-cutattribution of an error to a single bit is possible.

One possible modulation method for providing the code data word which isbased on the data word is a so-called DPSK (differential phase shiftkeying) method. In this method, one bit of the data word is in each casemodulated by the DPSK method with a carrier frequency for the data to betransmitted within the communications system. The DPSK method provides aphase jump of the carrier frequency in dependence on the data to betransmitted. Each data word is assigned two different DPSK code values.Accordingly, that two-bit string consists of either the bit values “10”or “01”. A respective string of these two possible single bits dependson the bit value of the respective code word bit to be modulated orassigned.

For possible implementation of a suitable modulation of the data word,two different DPSK methods will be presented. In the case of modulationperformed using the DPSK method, each individual bit of the data word isrepresented in two parts of differing level which may be represented,for example, by zeros and ones. Thus, transmission of the data or datawords with a constant frequency signal is possible.

In a first DPSK method, which is also referred to as the original DPSKmethod, the data word is modulated by performing a phase jump of aconstant frequency signal of the carrier frequency in dependence on thedata word bit to be transmitted, wherein a bit string of the code dataword to be provided in that case is not changed if the following bitvalue of the data word is a first of two possible bit values, forexample zero, and wherein the bit string is inverted if the followingbit value of the data word is a second of two possible bit values, forexample a one.

Instead of transmitting merely the two possible bit values of the dataword, it is always the carrier frequency, a “10” string, whetherinverted or non-inverted, that is transmitted. As long as a bit valuewithin the data word is a zero, the bit string is adopted unchanged fromthe preceding bit string. If the next bit value within the data word isa one, the sequence of the bit values is inverted, for example a “10”string is inverted to give a “01” string and vice versa. This may berepresented by a generation rule. That generation rule contains an EXORoperation (antivalence), which is represented here by the # symbol,between the carrier frequency T(t) and a stored binary value B(t) attime t. Let the stored binary value be at the beginning for t=0 B(t)=0.D(t) denotes a bit value of a bit string of the data word. M(t) denotesa modulated signal. The stored binary value is changed with every changein the bit values within the data word:B(t+1)=B(t)#(D(t)#D(t+1)) andM(t+1)=B(t+1)#T(t+1) orM(t)=B(t)#T(t).

A characteristic of the signal thus modulated, or of the code data wordthereby provided, is that in the case of a two-bit string, assigned toone bit of the data word, within the code data word a switch must alwaystake place in the middle, so that only the two-bit strings “10” or “01”are possible.

Alternatively, it is possible for a second DPSK method, which is alsoreferred to as a modified DPSK method or PSK method, to be used as amodulation method for converting the data word into the code data word.One of the two possible bit values of the data word is in each caseassigned one of the two possible two-bit strings of the code data word.In this case, the bit of the data word is assigned a first of twopossible two-bit strings if the bit to be modulated has a first of twopossible bit values within the code data word. If the data word bit tobe modulated has a second of two possible bit values, it is assignedwithin the code data word a second of the two possible two-bit strings.

This may mean that the bit value “0” of the data word is assigned thetwo-bit string “10” and the bit value “1” is assigned the two-bit string“01”. A converse, inverted assignment rule is also conceivable. Thatprocedure may also be represented by a generation rule in which an EXORoperation between the carrier frequency and one bit of the data word ineach case is taken into consideration:M(t)=D(t)#T(t).

Here also, a characteristic of the modulated signal resides in itsalways being necessary for a switch, “10” or “01”, to take place in themiddle of a two-bit value assigned to a bit of the data word.

Table 1 shows, by reference to an example, in a first line the time t.D(t), in the second line, denotes a predefined bit value of a bit stringof a data word. In a third and fourth line, the carrier frequency T(t)and a binary value B(t), respectively, at the time t are noted.M(t)_(ORIG) in the fifth line shows values of the code data wordgenerated by the original DPSK method. M(t)_(MOD) in the sixth lineshows values for a data word generated by the modified DPSK method.

TABLE 1 t 1 2 3 4 5 6 7 8 9 10 11 12 13 14 D(t) 0 1 0 0 1 1 1 0 0 1 1 00 0 T(t) 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 B(t) 00 1 1 1 1 1 1 0 0 1 1 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 M(t) 0 1 1 0 1 0 10 0 1 1 0 0 1 0 1 0 1 1 0 0 1 0 1 0 1 0 1 orig M(t) 0 1 1 0 0 1 0 1 1 01 0 1 0 0 1 0 1 1 0 1 0 0 1 0 1 0 1 mod

By using the two possible DPSK methods a clear-cut structure is imposedon the transmitted code data word, so that each two-bit string withinthe code data word consists of two different single-bit values. If asingle-bit value were to be corrupted during the transmission, a two-bitstring consisting of two identical single-bit values, either “00” or“11” would result. Thus, an erroneous two-bit string within the codedata word is identifiable in a clear-cut manner.

The system according to the present invention is configured to carry outerror handling for a datum to be transmitted between at least twostations. The system has a first and at least one second station.

In this system, a first station is configured to generate for the datum,in accordance with a predefinable coding rule, at least two data wordsconsisting of bits. In this instance, depending on the value of arunning digital sum formed over the data words in each case, one of thetwo code data words, which corresponds to the same datum, is selected.

In addition, the first station calculates a first running digital sum(RDS). The latter may, for example, correspond to the running digitalsum of the selected data word or may be formed by adding the runningdigital sum of the selected data word to a currently given runningdigital sum. The selected data word is converted by the first stationinto a code data word, wherein a bit of the data word is in each case tobe assigned a two-bit string with two different bit values. The firststation transmits the code data word and the first running digital sumto the at least one second station.

In one possible embodiment, for calculation of the first running digitalsum in the case of a plurality of data words of a group, all runningdigital sums of the plurality of data words are to be added up to atotal value giving the first running digital sum. The group with thecode data words resulting from the data words is referred to as a dataframe. The first running digital sum for all data words of the groupbelonging to the data frame is transmitted by the first station to atleast one second station within the system preferably periodically afterthat data frame.

The at least one second station examines a received code data word toascertain whether it has a two-bit string with two identical bit values.If that is the case, the two-bit string with the two identicalsingle-bit values has been transmitted between the stations of thesystem incorrectly.

Taking the first running digital sum into consideration, a single bit ofthe code data word having a single error may be identified andcorrected. If a two-bit string has an error, it appears as a sequence ofidentical single-bit values, either as “00” or as “11”. Such anerroneous two-bit string may have corresponded to the originallyerror-free two-bit string “01” or “10”. On demodulation, there areaccordingly obtained two alternative demodulated data words of whichprecisely one may be identified, on the basis of its respective runningdigital sum, as the data word originally to be transmitted.

The computer program according to the present invention having programcode means is provided to perform all the steps of the method accordingto the present invention when the computer program is executed on acomputer or a corresponding computational unit, e.g., in the systemaccording to the present invention or in a computational unit within thesystem according to the present invention.

The computer program product according to the present invention havingprogram code means that are stored on a computer-readable data medium isprovided to perform all the steps of the method according to the presentinvention when the computer program is executed on a computer or acorresponding computational unit, e.g., on the system according to thepresent invention or a computational unit in the system according to thepresent invention.

It will be appreciated that the features mentioned hereinbefore and thefeatures described hereinafter may be used not only in the combinationgiven in each case but also in other combinations or individually,without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration of one example embodiment of a systemaccording to the present invention, with consideration being given toindividual steps of one example embodiment of a method according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The system 1 shown schematically in FIG. 1 includes a first station 3,provided as a transmitter, and a second station 5, provided as areceiver. Between first station 3 and second station 5, a datum D is tobe transmitted as information. Datum D may be a number, an alphabeticcharacter or any other character.

To carry out the method, datum D is to be encoded, datum D beingassigned a data word DW consisting of bits, that is, two clearlydistinguishable characters, for example ones and zeros. That data wordmay be selected in accordance with a predefinable coding rule 7 from twopossible data words, a first data word DWA and a second data word DWBwhich is the inverse of the first data word. Coding rule 7 provides botha clear-cut encryption of the at least one datum D and a clear-cutdecryption from one of the possible data words DWA or DWB to datum D.

A calculation 9 then takes place, in which a running digital sum isformed over the two data words DWA and DWB in each case. Data words DWAand DWB have a specific number z of characters, the first of the twopossible bit values, for example a one, occurring a times within dataword DWA or DWB. Accordingly, the second of the two possible bit values,for example the zero, occurs (z-a) times. The running digital sum iscalculated from the difference between the first sum a for a totalnumber of the first bit value and a second sum (z-a) for a total numberof the second bit value within data words DWA and DWB. The runningdigital sum is in this case: (2a-z) or, depending on the definition ofthe running digital sum, the negative value thereof (z-2a).

At selection 11 of data word DW from the two possible data words DWA andDWB for datum D it is also possible to take into consideration acurrently given running digital sum. The aim in so doing may be forfirst running digital sum S1 to have as small as possible a value inmagnitude and, ideally, to be zero. It is from that perspective thateither the first or the second assignable data word DWA or DWB isselected for datum D as the actual data word DW provided for furtherprocessing.

In addition, it is provided that the selected data word DW be convertedby a modulation method 13 into a code data word CDW. In that operation,a bit of data word DW is in each case assigned a two-bit string formedfrom two different single-bit values. Possible modulation methods 13that may be provided for the present method are so-called DPSK methods.

Transmission 15 of code data word CDW and of first running digital sumS1 from first station 3 to second station 5 of system 1 then takesplace. That transmission may take place periodically. The possibilitythat a single bit of code data word CDW sent from first station 3 may becorrupted during transmission 15 cannot be ruled out. A code data wordCDW′ received by second station 5 may therefore differ from code dataword CDW sent.

If a two-bit string has an error, it appears as a sequence of identicalsingle-bit values, either as “00” or as “11”. Such an erroneous two-bitstring may have corresponded to the original error-free two-bit string“01” or “10”. On demodulation 17, there are accordingly obtained twoalternative demodulated data words of which precisely one may beidentified, on the basis of its respective running digital sum, as thedata word originally to be transmitted. The calculation of a respectivesecond digital sum S2 for the two alternative demodulated data words iscarried out in this case analogously to the calculation of first runningdigital sum S1.

Received code data word CDW′ is subjected to an examination procedure19. The latter examines whether that received code data word CDW′ has atwo-bit string with two identical single-bit values. Examinationprocedure 15 is carried out taking account of the fact that the originalcode data word CDW sent by first station 3 must have, in accordance withmodulation method 13, two-bit strings with two different single-bitvalues. In addition, in the case where a single-bit error has occurred,there are obtained, as already explained, two alternative demodulateddata words with correspondingly two different second digital sums S2. Inexamination procedure 19, first running digital sum S1 is compared withthe respective second digital sums S2. That provides a further criterionfor the exact identification of a single-bit error that has occurred.

Thus, two cases are obtained for received code data word CDW′. If, inaccordance with a first case, it is found in examination procedure 19that received code data word CDW′ does not have a two-bit string withtwo identical single-bit values, then code data word CDW originally sentwas sent correctly under that proviso between the two stations 3 and 5,and therefore received code data word CDW′ corresponds to code data wordCDW sent. Thus, it is possible to reconstruct from that correct receivedcode data word CDW′ by demodulation 17 data word DW and from that,taking into consideration coding rule 7, original datum D.

If it is found in examination procedure 19 that a two-bit string ofreceived code data word CDW′ has two identical single-bit values, thenthat two-bit string has an error or single-bit error. This means that atwo-bit string of received code data word CDW′ consists of two identicalsingle-bit values, for example two zeros or ones. Clear-cut attributionof the error and hence a correction 21 of one of the two single-bitvalues of the erroneous two-bit string is carried out taking intoconsideration first running digital sum S1 which was also transmitted.First running digital sum S1 provides information on how often the twobit values have occurred within original data word DW. Taking firstrunning digital sum S1 into consideration, it is possible to assign tothe erroneous two-bit string of received code data word CDW′ the correctoriginal bit value of data word DW.

One possible way of implementing this example embodiment of the methodwill be described with reference to a concrete example. In accordancewith the predefinable coding rule, a given datum D is assigned a firstdata word DWA=“010010” and a second data word, which is the inversethereof, DWB=“101101”. A value for a final bit of those two data wordsDWA and DWB indicates whether the data word has been selected from theset of inverted or non-inverted data words. If, for example, thecurrently given running digital sum was S1 ₀=4, and the first runningdigital sum is given by the sum of the currently given running digitalsum and the running digital sum of the data word to be selected, firstrunning digital sum S1 becomes, on consideration of first data word DWA,S1=2 and, on consideration of second data word DWB, S1=6. Thecalculation of the first running digital sum corresponds in the presentcase to the difference between the number of ones occurring and thenumber of zeros occurring. When those respective running digital sums soobtained are added to the currently given running digital sum S1 ₀=4,the respective values mentioned are obtained. In order, for example, tokeep the magnitude of first running digital sum S1 as small as possible,first data word DWA is accordingly selected as data word DW andmodulated to form a code data word CDW.

Assume there is provided as modulation method 13 in the present examplea so-called second, modified DPSK method. According to that modifiedDPSK method, a single-bit value D(t) is assigned, in accordance with ageneration rule: M(t)=D(t)#T(t), by way of an EXOR operator # betweenthe carrier frequency T(t) and that single-bit value D(t), a two-bitstring M(t) and thus a modulated signal. Without limitation of thegenerality, a bit value “0” of data word DW is assigned the two-bitstring “10”. The bit value “1” of data word DW is accordingly assignedthe inverted two-bit string “01”.

Calculation 9 of first running digital sum S1 would in this case resultin S1=2, since the bit value “0” occurs four times and the bit value “1”occurs twice within data word DW, which gives a running digital sum of“−2” and, on addition to the mentioned currently given running digitalsum S1 ₀=4, ultimately results in S1=2.

Owing to modulation method 13, there is obtained for data word DW a codedata word CDW=“10 01 10 10 01 10”.

Alternatively, modulation method 13 may also be carried out with theso-called original DPSK method. In that case, a bit string of a codedata word CDW is not changed if the subsequent bit value of data word DWis equal to zero. The bit string of code data word CDW is inverted, onthe other hand, if the subsequent bit value of data word DW is equal toone.

Second station 5 receives, after transmission 15 of sent original codedata word CDW, a corresponding code data word CDW′ which may or may nothave been corrupted by transmission 15. During examination procedure 19,that received code data word CDW′ is examined to ascertain whether ithas a two-bit string with two identical single-bit values, that is, atwo-bit string consisting either of the single-bit values “00” or “11”.If that is the case, within an erroneous two-bit string of received codedata word CDW′ the erroneous single bit is identified and corrected byexamining the coding rule and first running digital sum S1.

The present example assumes that during transmission 15 the first singlebit of the second two-bit string was corrupted, with the result thatsecond station 5 has received the following code data word CDW′=“10 1110 10 01 10”. On the basis of the predefined structure of code data wordCDW sent and of code data word CDW′ received, it is possible torecognize in a simple manner that the second two-bit string of code dataword CDW′ has been transmitted incorrectly.

Whether the first single bit of that second two-bit string has beencorrupted, and therefore the original two-bit string is “01”, or whetherthe second single bit of that two-bit string has been corrupted, andtherefore the original two-bit string is “10”, may be traced by takingfirst running digital sum S1 into consideration.

The erroneous code data word CDW′=“10 11 10 10 01 10” may haveoriginated from a string CDW′₁=“10 10 10 10 01 10” or CDW′₂=“10 01 10 1001 10”. Demodulation of CDW′₁ gives DW′₁=“0 0 0 0 1 0” with a runningdigital sum of “−4” which, when added to the current given runningdigital sum S1 ₀=4, gives a second running digital sum S2=0.Demodulation of CDW′₂ gives DW′₂=“0 1 0 0 1 0” with a running digitalsum of “−2” which, when added to the current given running digital sumS1 ₀=4, gives a second running digital sum S2=2. On comparison of therespectively obtained second running digital sum S2 with the transmittedfirst running digital sum S1, it is apparent without any ambiguity thatDW′₂ corresponds to the data word DW that was originally to betransmitted, and the error that occurred could accordingly be identifiedand corrected.

1. A method for error handling in transmission of a datum over acommunications system, comprising: generating at least two data wordsconsisting of bits for the datum in accordance with a predefined codingrule; selecting one of the at least two generated data words taking intoconsideration a running digital sum formed over the corresponding dataword, wherein the running digital sum of the selected data word is usedfor formation of a first running digital sum; converting the selecteddata word by a modulation method into a code data word, wherein each bitof the selected data word is assigned a two-bit string with twodifferent single-bit values; transmitting the code data word and thefirst running digital sum; analyzing the received code data word toascertain an error, wherein an error is determined if a two-bit stringwith two identical single-bit values occurs in the received code dataword; and if an error is determined, correcting the error using at leastthe first running digital sum.
 2. The method as recited in claim 1,wherein the selection of one of the at least two generated data words issubject to the condition that the first running digital sum has as smalla magnitude as possible.
 3. The method as recited in claim 1, whereinthe at least two data words generated in accordance with the predefinedcoding rule are inverse of each other.
 4. The method as recited in claim1, wherein an erroneous single bit of the received code data word isidentified and corrected by taking into consideration the predefinedcoding rule.
 5. The method as recited in claim 1, further comprising: ifan erroneous two-bit string with two identical single-bit values occursin the received code data word, converting the received code data wordinto at least two data words by a demodulation method corresponding tothe modulation method; calculating a running digital sum for the atleast two data words formed by the demodulation method, wherein therunning digital sum is used for the formation of a second runningdigital sum for each of the at least two data words formed by thedemodulation method; and comparing each of the second running digitalsums with the received first running digital sum to identify and correctthe error.
 6. The method as recited in claim 1, wherein the selecteddata word is modulated by a first differential phase shift keying methodby: (a) not changing a bit string of the code data word if a followingbit value of the selected data word has a first of two possible bitvalues; and (b) inverting the bit string of the code data word if thefollowing bit value of the selected data word has a second of twopossible bit values.
 7. The method as recited in claim 1, wherein theselected data word is modulated by a second differential phase shiftkeying method by: (a) assigning to the two-bit string in the code dataword for a bit to be modulated in the selected data word a first of twopossible bit strings, if the bit to be modulated in the selected dataword has a zero as the bit value; and (b) assigning to the two-bitstring in the code data word for a bit to be modulated in the selecteddata word a second of two possible bit strings, if the bit to bemodulated in the selected data word has a one as the bit value.
 8. Themethod as recited in claim 5, wherein the code data word and the firstrunning digital sum are sent by a first station and received by at leastone second station, and wherein the second running digital sum iscalculated by the at least one second station, and wherein an erroneousbit of the code data word is recognized and corrected by the at leastone second station.
 9. A system configured to perform error handling fora datum transmitted between at least two stations, comprising: a firststation configured to: a) generate at least two data words consisting ofbits for the datum in accordance with a predefined coding rule; b)select one of the at least two generated data words taking intoconsideration a running digital sum formed over the corresponding dataword, wherein the running digital sum of the selected data word is usedfor formation of a first running digital sum; c) convert the selecteddata word by a modulation method into a code data word, wherein each bitof the selected data word is assigned a two-bit string with twodifferent single-bit values, and d) transmit the code data word and thefirst running digital sum; a second station configured to: a) receivethe code data word and the first running digital sum transmitted by thefirst station; b) analyze the received code data word to ascertain anerror, wherein an error is determined if a two-bit string with twoidentical single-bit values occurs in the received code data word; andc) if an error is determined, correct the error using at least the firstrunning digital sum.
 10. A computer-readable storage medium storing acomputer program having program codes which, when executed on acomputer, controls a method for error handling in transmission of adatum over a communications system, the method comprising: generating atleast two data words consisting of bits for the datum in accordance witha predefined coding rule; selecting one of the at least two generateddata words taking into consideration a running digital sum formed overthe corresponding data word, wherein the running digital sum of theselected data word is used for formation of a first running digital sum;converting the selected data word by a modulation method into a codedata word, wherein each bit of the selected data word is assigned atwo-bit string with two different single-bit values; transmitting thecode data word and the first running digital sum; analyzing the receivedcode data word to ascertain an error, wherein an error is determined ifa two-bit string with two identical single-bit values occurs in thereceived code data word; and if an error is determined, correcting theerror using at least the first running digital sum.